Moved the sprite sheet management for Mermaid (Lorelei) into ComboState.

It is currently only using her default sprite sheet, but if supplied a list of sprite sheets as argument 2,
it can hold more.

ComboState can also be used for more than just her - other sprites can have it too. This means that other
sprites can perform combos once this is fleshed out completely!!!
Pressing Space Bar now enters a dummy combo state (reused main sprite sheet)
Fixed some bugs with combo state. ComboState constructor arguments rearranged. Please see comment on __init__
I'm now passing in the dark male mermaid sprite sheet as a combo state. Taking damage will interrupt the combo
state and undo the sex change :).